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An intense and dynamic winter storm system is affecting the Southeast U.S. today. Most of the rain and storms are west of the MS River or north of I-40 right now. Much of the heavy precip in Missouri and Illinois is snow! But note the storms exploding over SE Texas, Louisiana, and SW Mississippi. That will be the big trouble over LA/MS/AL today. Check out the current surface map (annotated).

Courtesy UCAR

So, it is 70 degrees in Muscle Shoals, AL but 42 in Memphis and 28 and Springfield, MO. Very strong cold front. The surface low that has formed near the LA/AR border (note the wind flow swirlling around it) could be a big troublemaker for us today. Upper air charts below show a jet to our north at 500 mb (18,000 feet), and divergence due to atmospheric adjustment south of the jet is causing the surface low. Also, at 850 mb (about 5,000 feet), note the strong southerly flow developing, transporting warm moist air into the Southeast U.S.

Maps courtesy pivotalweather.com

The map below shows the trajectories of air that will be located at various points by 2100 GMT today (3 pm CST). Note that the air in Birmingham, AL comes from SW Florida between Tampa and Ft. Myers.

Courtesy NOAA

So, as the afternoon goes on, temperatures from central and south MS into north and central AL, especially with the sunshine we are getting, will warm into the 70s, and dewpoints will slowly climb into the mid 60s, setting up a prime lower atmosphere for thunderstorm development. One thing that will hold back storms over eastern MS and most of AL through late afternoon is a cap of warm air aloft, shown on this morning’s National Weather Service weather balloon at Calera, AL.

With time, as the cold front approaches, upward motion and cooling aloft, along with warming at the surface, will erode that cap. That will happen quickly over LA and western MS this afternoon, so storms will likely develop in that area quickly over the next 3 hours, and SPC has issued a Tornado Watch over that region. It will take longer over east MS, and maybe until the cold front actually arrives this evening in central AL, to fully become unstable. By then, it will be dark, surface temperatures a tad cooler, so the air over AL will not become as unstable as that in MS. The greatest risk for tornadoes today will be over Louisiana and Missisippi. There is a chance a few isolated supercell storms with a tornado threat could form west of I-65 over Alabama between 3 and 6 pm, but the widespread severe weather in Alabama will arrive after sunset.

As the cold front moves into Alabama this evening, the air will become more unstable. CAPE values will reach 500-800 J/kg, nothing like the 1500 J/kg in south MS, but enough for severe storms. The concern is the wind shear associated with the surface low. The wind will change direction and increase in speed quickly with height, creating 0-1 km SRH values near 300 m2/s2 over central Alabama. This wind shear provides the energy for storm rotation. A good combination parameter for wind shear and instability, the EHI, is shown below for 00, 03, and 06 GMT (6 pm, 9 pm, and 12 am CST).

Maps courtesy pivotalweather.com

Note the EHI decreases overall as it moves into Alabama this evening, mainly due to the wind shear pulling away to the north but the instability staying south.

The bottom line here is that there will be a threat for tornadoes, some significant, from Louisiana into Mississippi this afternoon and early evening. As the dynamics pull away to the north and it gets dark, storms will likely grow into a squall line as they move into Alabama around 7 pm and into BHM by 9 pm. However, with the high wind shear, straight-line wind damage and isolated tornadoes will likely occur along the line across west and central Alabama this evening, and in west Alabama, a couple of supercells could form ahead of the line with a higher tornado threat in places like Hamilton, Tuscaloosa, and Demopolis.

This is a dangerous weather situation, especially in Louisiana and Mississippi, but even into Alabama. Review your tornado safety plan wherever you will be. If a Tornado Warning is issued, go to the lowest floor of the building you are in, get in a small room near the center of the building (away from walls, doors, garage doors, and outside windows), get under something sturdy if possible like a desk or table, and if you have helmets (baseball, football, bike, etc) wear them.

The Arctic cold front has entered north Alabama, with winds turning to NW and temperatures falling quickly into 40s. Deeper into the cold air but not far away, it is 36 in Oxford, MS and 21 in Fayetteville, AR. That air is rushing toward Alabama partially to its own weight, at wind speeds 15-35 mph. The cold front will reach BHM, TCL, and GAD around 11 pm, and temperatures will go below freezing around 6 am.

Radar shows rain along and behind the front. There has been a lot of talk about snow and ice for tomorrow morning’s rush hour in central Alabama. It is difficult for precipitation to survive in an area of cold advection, where cold air is replacing warm air and therefore flows downhill slightly, drying up precipitation. In addition, the cold air will come in shallow,, so there will be a warm nose aloft preventing snow from reaching the ground (see figure).

The problem COULD BE a brief window tomorrow morning where there is still precipitation, likely rain, falling out of it, then reaching the ground and freezing, causing some icy bridges. This is most likely in north Alabama, but is even a possibility as far SE as I-59. Another possibility is that we get 1/2″ of rain overnight, and the cold air comes in so fast some of the water puddles freeze on a few bridges. The thing helping us out on this is the very warm ground temperatures, thanks to sunshine yesterday and BHM reaching 72 degrees today. I don’t see this as a big deal, but you definitely want to check weather in the morning before leaving home, and be careful on bridges and higher elevation roads.

The cold air will be downright crazy! Temperatures will hover between 30 and 35 degrees all day tomorrow in BHM, and with northwest winds 15-30 mph, wind chills will range between 5 and 20 degrees during the day! We will drop slowly through the 20s tomorrow night, ending up near 22 degrees Tuesday morning. We will be below freezing for 24 out of 28 hours from 6 am Tue through 10 am Wed, and 17 hours straight. Remember pets, plants, pipes. And most of all, the elderly or disabled who may not have access to safe, sufficient heat.

Above is a map of current temperatures across North America. The large purple area covering much of central Canada, the Arctic Ocean, and Greenland, has temperatures below zero now, and some are below -20 F. We didn’t get any significant cold air flush outs in October (when it was still going above 100 F in parts of Alabama, Mississippi, and Georgia), but the sun angle has been low up in the Arctic for a while now and the days very short (0-7 hours long). So, cold air has really built up.

Over the next 2-3 days, a large upper-level ridge will build into Alaska, and the upper-level convergence on its east side will produce a large high pressure area in western Canada. Cold air is more dense and therefore heavier than warm air, and that will intensify the developing high pressure area. The shallow cold air will start to try to flow out of the dome of high pressure, but it will be blocked to its west by the Rocky Mountains, so it will be forced southeast into the U.S. starting this weekend.

This is a very large and cold air mass. So its own weight, combined with the favorable upper-level ridge to the west keeping pressures high, and a surface low pressure area forming near the Great Lakes, will push this Arctic air deep into the central and eastern United States. By Tuesday evening, temperatures at 850 mb (about 5,000 feet) will be as much as 21 C (nearly 40 F) below normal for mid-November across most of the U.S. east of the Rocky Mtns.

A significant cold front will move through the Southeast US Thursday and Thursday night, with lows dropping below freezing north of I-20 (places like Little Rock, Nashville, Huntsville, and Chattanooga), and high temperatures Friday into the weekend will only be in the 40s and 50s over many areas. But this is not the main event. That moves through next Monday and Tuesday. Below is a graph of our predicted temperatures every 6 hours at Birmingham airport.

So, here we sit today near 70 degrees, we will drop to near freezing by Friday morning, have a normal, chilly weekend, then the cold blast will come Monday night. Temperatures will drop very quickly from the 50s in the early evening to the lower 30s by Tuesday morning, stay in the the 30s all day on Tuesday (with wind chills during the day in the 20s), then drop to near 20 degrees by Wednesday morning.

We have high confidence in this cold outbreak because of the ridge over Alaska and the large pool of cold air over the Arctic and Canada. Plus, all 3 major computer models (the American GFS, the Canadian GDPS, and the European ECMWF) all show this happening. A zoom-in on ECMWF model projected temperatures around the Birmingham metro ares on Wednesday morning is shown below.

ECMWF predicted temps Wed 6 am.

We didn’t have that much cold air last Winter, and if this pattern were to set up again in December or January, we could easily see temperatures near zero here in Alabama. With this cold outbreak next week, some locations in Arkansas and Tennessee may drop into the single digits, and a little snow is even possible, especially in the mountains of east Tennessee and western NC. This will be the coldest air since January 30, 2019 here in Alabama.

Interesting to note that the temperature hit 101 degrees on October 3 in Birmingham. If we drop to 21 next week, that will be an 80 degree drop in temperature in less than 45 days. This large of a temperature drop in 45 days has only occurred 5 other times since 1900.

This is the kind of cold air outbreak than can freeze pipes, kill plants, and harm animals. Go ahead and think about winterization around the home, such as covering outdoor faucets, draining water hoses, and winterizing boats. Make sure your antifreeze is good in your car and your tire pressure is sufficient (it will drop significantly next week). Once the cold air arrives, if you want any plants to live, bring them inside or cover them. Also remember that it is sometimes up to 5 degrees colder right near the ground than the official 2 meter temperature, so bring pets inside or at least make sure they have plenty of warm stuff to use inside a dog house, and dog house doors should never face north or west. Check on any elderly or disabled people you think might not have sufficient, safe heat. And check your own heating system if it hasn’t run this year.

The biggest cold front of the year so far will move through Alabama on Thursday. Temperatures will drop very quickly during the day, starting out near 70 at 7 am, drop to 55 by 10 am, then into the 40s during the afternoon. It will feel even colder, with northwest winds 10 to 20 mph. It will be about 25 degrees colder when you get home tomorrow evening than when you leave tomorrow morning, so dress appropriately!

Temperatures across the Southeast US show the strong cold front right now (1:45 pm CDT). It is near 70 across much of Alabama and Mississippi, but behind the cold front it is only in the 50s in central Arkansas, 40s in Fayetteville and Fort Smith, AR, then 30s in Oklahoma.

The cold front will tighten up overnight. Take a look at the computer model based temperature chart for tomorrow morning at 7 am. That is a huge temperature change over a short distance.

We may some light rain tomorrow, but it will likely move out in time for kids’ trick or treat activities. But it will be cold, so bundle them up! At 8 pm, expect 42 degrees with a wind chill in the 30s.

Early mornings this weekend we will get close to freezing, and many locations north of I-20 will get their first freeze of the year this weekend. There will probably be enough wind to prevent frost on Friday morning, but frost is possible Saturday and Sunday mornings.

The above chart shows the 120-year average high temperatures in Birmingham, AL (black), along with the 1-standard deviation bands (where about 70% of all high temperatures should fall on a given date). Through most of 2019, temperatures stayed within these bands most of the time, so it was a normal year. Even the 100 degree days in August were not too far outside the normal. Note how the bands are tightly packed in the summer, meaning there is not that much variance from the normal in the Summer. The bands get much wider in the cold season. In late December, for example, high temperatures may be as low as the 20s or as high as the 70s!

But then, in September, we went into a very dry pattern, no cold fronts came through, and temperatures stayed very hot. Droughts and heat waves go together…the sun heats the ground which then heats the air, and if there is no water to evaporate, all of the sun’s energy goes into heating.

September 2019 was the 2nd warmest September on record with an average high of 94.5 and average temperature of 82.1. Only Sep 1925 was hotter. Sep 2019 was the 21st warmest month on record, period, beating out dozens of Julys and Augusts.

Using Coleman and Knupp, LLC’s 7-day moving average for all days going back decades, the expected distribution of high temperatures (and low temperatures) can be shown for any day of the year, in any city, with expanded precision. Below is the distribution for Oct 1, yesterday, for Birmingham. It is close to a normal distribution, with a median of 81 degrees, mode 83 degrees, standard deviation 7 degrees. We hit 99 yesterday, something that had never happened during the period Sep 29-Oct 4 before, so obviously it was a record high temperature.

But, using climate data from any site, we at Coleman and Knupp, LLC can custom design the probabilities of exceeding or going below any high or low temperature at any location, and the probability of given amounts of rain. This can be useful for planning weddings and other outdoor events beyond 2 weeks out, when we have essentially no weather predictive capability.

The good news is that a cool front may touch off come clouds and showers this weekend, cooling us off a few degrees, then a more significant cold front will sweep through early next week, making it feel a bit more like October. And, as the sun’s angle and the length of day are both decreasing rapidly now, the Northern Hemisphere has to cool off and more cold fronts will come to the eastern U.S.! Below I show the length of day at Birmingham across the year (solid), and the rate of change (dashed). Right now, our days are just under 12 hours long, and we are losing about 2 minutes of daylight every day. The Geochron-style image shows that the sun is illuminating more of the Southern Hemisphere than the Northern one now, too.

A bona fide cold front is moving southward through the United States today, and will move all the way through most of Alabama, Mississippi, Tennessee, and Georgia tomorrow!

The term “cold front” is always used to describe the front edge of cool air masses that replace warmer ones. Of course, it is not going to get “cold” here in the Southeast U.S., but it will certainly feel much more pleasant than the sauna we have felt the past 3 weeks, with morning lows dropping near 60 degrees as far south as Birmingham and Atlanta, and daytime high temperatures only in the mid 80s.

Current temperatures (1 pm CDT)

The above map shows current temperatures from SE Kansas to NW Alabama right now. Some of the lower temperatures over parts of KY and AR are being affected by rain. But, it is 79 in Joplin, MO at 1 pm, and they have sunshine! So, this is a legitimate cool air mass. By tomorrow afternoon, winds will turn out of the north along I-20 from Jackson to Birmingham to Atlanta, and the cooler air will be moving in.

The air in places like Birmingham and Nashville will be coming from the eastern Great Lakes, as the NOAA computer model trajectories show below.

We will see some rain showers and maybe a few thunderstorms as the front moves through overnight and tomorrow morning. But, from tomorrow afternoon through Thursday, temperatures will be below normal. Some locations in the colder valleys of northeast AL, north Georgia, and east TN will easily drop well into the 50s.

Another big change that will make it feel nice will be the much lower humidity. Dewpoint is a meteorologist’s favorite measure of water vapor in the atmosphere because it is not affected by temperature, it simply indicates how much water vapor is in the air. The dewpoints over much of the Southeast U.S. the past few weeks have been between around 73, indicating a water vapor content of 17.5 grams of water vapor for every kg of air. Tuesday through Friday, dewpoints will be in the 50s, indicating water vapor contents dropping by 50%, to about 9 grams per kg. You will feel the difference!

Enjoy it, because with the strong, nearly overhead sunshine angle of July and August and warm soil, it won’t last, and we should be back into the 90s by the weekend. The humidity will come back too, with a big Gulf of Mexico nearby and water temperatures in the 80s.

Barry is still considered a tropical storm right now by NHC, even though its maximum winds are out over open waters of the Gulf of Mexico. These winds are far from the center of circulation and lowest pressure near Shreveport, Louisiana, where winds are slowed by friction and thunderstorms are not as intense. Winds over most of south Louisiana as of 2 pm had dropped to 25-35 mph.

The big story with Barry now is the rain it is producing. The regional radar composite above shows a near solid wall of rain over the state of Mississippi, extending into much of eastern Louisiana and eastern Arkansas. Along the eastern edge of the area of heaviest rain, bands of thunderstorms are forming over parts of western and southern Alabama, and these may spread into central Alabama later today. There are also a few bands of thunderstorms over northern Georgia.

Some parts of south MS, extreme south AL, and southwest LA have already received 3-8″ of rainfall from Barry, and some rivers are already flooding. As Barry’s pressure continues to rise and the circulation weakens over the next 48 hours, the mass of rain will slowly decrease in size and intensity. But, even by Tuesday, with tropical moisture in place over the Southeast US, scattered afternoon storms will be increased, and some heavy rainfall amounts will occur from Memphis to Nashville and Paducah. The NOAA total rainfall forecast for the next 7 days reflects this increase centered near Memphis, with amounts over 7″ centered there, and amounts over 4″ expected in Little Rock, Paducah, and Jackson, TN. Rainfall amounts will drop off dramatically to the east over AL, middle and eastern TN, and GA, where less than 2″ is expected. Given the alignment of the rain maximum, some flooding along the Mississippi River, especially from Cape Girardeau south, is likely.

NOAA forecast 7-day rainfall starting now

One more interesting thing being picked up by upper-air weather balloons and computer models is the warm air aloft associated with Barry. Even though it has weakened to tropical storm winds, one can still see that temperatures at 500 mb (about 18,000 feet) are 10 to 15 degrees warmer above Barry than at other locations even over the Southern United States and the Gulf of Mexico. It is this warm air aloft, produced by the latent heat release in thunderstorms, that causes the air aloft to be lighter, and therefore cause the low pressure at the surface that drives tropical systems like hurricanes.